1. Field of the Invention
The present invention relates to a process for catalytically dewaxing lube oils. In particular the invention relates to a process for producing lube oil having an increased viscosity index by contacting a solvent extracted hydrocarbonaceous oil feedstock with a silicoaluminophosphate molecular sieve.
2. Description of the Related Art
In general, the basic premise in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock. The lubricant stock should have a predetermined set of properties, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. Current trends in the design of automotive engines are associated with higher operating temperatures as the efficiency of the engines increases. These higher operating temperatures require successively higher quality lubricants; one requirement is for higher viscosity indices (V.I.) in the lube oil in order to reduce the effects of the higher operating temperatures on the viscosity of the engine lubricants.
Viscosity index indicates the degree of change of viscosity with temperature. A high viscosity index of 100 indicates an oil that does not tend to become viscous at low temperature or become thin at high temperatures. For purposes of the present invention, whenever V.I. is referred to it is meant the V.I. as determined by ASTM D-2270.
Mineral oil based lubricants are conventionally produced by a set of subtractive unit operations to isolate the lubricant stock and to remove unwanted components from the oil. For the preparation of a high grade distillate lubricating oil stock, the current practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This provides one or more raw stocks having a boiling range of about 350.degree. to about 1050.degree. F. This is then further separated, under vacuum processes, into suitable boiling range distillate fractions (gas oils) and a residual fraction which, after deasphalting and severe solvent treatment may also be used as a lubricant base stock usually referred to as a bright stock. The gas oils undergo solvent extraction, also known as solvent refining, to remove low viscosity index components to produce oils known as neutral oils. These solvent extracted neutral oils are also known as raffinates.
The raffinates are solvent dewaxed by cooling oil-solvent admixtures under controlled conditions for crystallization of the paraffinic wax present in the admixtures. In such processes, the raffinates, or mixtures of raffinates and dewaxing solvent, are heated to a temperature at which the wax is dissolved. The heated charge is then passed into a cooling zone wherein cooling is undertaken at a uniform slow rate in the range of about 1.degree. to 8.degree. F./min. (0.56.degree. to 4.4.degree. C./min) until a temperature is reached at which a substantial portion of the wax is crystallized and at which dewaxed oil product has a selected pour point temperature. Upon achieving the desired dewaxing temperature, the mixture of wax crystals, oil and solvent is subjected to solid-liquid separation for recovery of a wax free oil-solvent solution and a solid wax containing a minor proportion of oil. This solid wax/oil composition is known as slack-wax.
The separated oil-solvent solution is subjected to distillation for recovery of a solvent fraction and a dewaxed oil product fraction. A refined lubricant stock may be used as such as a lubricant, or it may be blended with another refined lubricant stock having different properties. Or the refined lubricant stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and V.I. improvers.
Slack wax may be recovered as is, or may be subjected to additional processing, such as repulp filtration for the removal of additional oil. Solid-liquid separation techniques which may be employed for separation of wax crystals from the oil-solvent solutions include known solid-liquid separation processes, such as gravity settling, centrifugation, and filtration. Most commonly, in commercial processes, filtration in a rotary vacuum filter, followed by solvent wash of the wax cake, is employed.
Solvents known to be useful as dewaxing solvents are the ketones containing 3 to 6 carbon atoms, for example, acetone, methylethylketone (MEK) and methylisobutylketone (MIBK); mixtures of ketones; and mixtures of ketones with aromatic hydrocarbons including benzene and toluene. Halogenated low molecular weight hydrocarbons, including dichloromethane and dichloroethane, and their mixtures are also known dewaxing solvents. Solvent dilution of waxy oil stocks maintains fluidity of the oil for facilitating easy handling, for obtaining optimum wax-oil separation, and for obtaining optimum dewaxed oil yields. The extent of solvent dilution depends upon the particular oil stocks and solvents used, the approach to filtration temperature in the cooling zone and the desired final ratio of solvent to oil in the separation zone.
Since processes which remove wax, such as solvent dewaxing, will give a low yield with very waxy feeds, catalytic dewaxing processes are preferred. Catalytic processes are more economical and lower the pour point of the waxy feedstock by selectively cracking the longer chain n-paraffins. A disadvantage associated with catalytically dewaxing a raffinate is that a number of useful products become degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may the lighter n-paraffins which do not contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it is desirable to limit the degree of cracking which takes place during a catalytic dewaxing process. Since lube oil is valuable, maximization of the yield is commercially important. Because the catalyst used in the process of this invention is very selective, the gas yield is reduced, thereby preserving the economic value of the feedstock.
U.S. Pat. No. 4,734,539 discloses a method for isomerizing a naphtha feed using an intermediate pore size zeolite catalyst, such as an H-offretite catalyst. U.S. Pat. No. 4,518,485 discloses a process for dewaxing a hydrocarbon feedstock containing paraffins by a hydrotreating and isomerization process.
U.S. Pat. No. 4,689,138 discloses an isomerization process for reducing the normal paraffin content of a hydrocarbon oil feedstock using a catalyst comprising an intermediate pore size silicoaluminophosphate molecular sieve containing a Group VIII metal component which is occluded in the crystals during growth.
U.S. Pat. No. 5,135,638 issued on Aug. 4, 1992 to Miller discloses a process for producing lube oil from a feedstock having greater than 50% wax. The feedstock is isomerized over a catalyst comprising a molecular sieve having generally oval 1-D pores having a minor axis between about 4.2 .ANG. and about 4.8 .ANG. and a minor axis between about 5.4 .ANG. and about 7.0 .ANG. and at least one Group VIII metal at a pressure of from about 15 psig to about 2000 psig.
U.S. Pat. No. 4,960,504 issued on Oct. 2, 1990 to Pellet et al. discloses a process for producing an oil having a reduced pour point by catalytically dewaxing the hydrocarbon feedstock using a catalyst comprising a silicoaluminophosphate and an inorganic oxide matrix. The patent does not indicate that it would be possible to produce a lube oil having an extra high V.I. from a solvent extracted feedstock.
U.S. Pat. No. 4,859,311 issued on Aug. 22, 1989 to Miller, the disclosure of which is incorporated herein by reference in its entirety, discloses a process for dewaxing a hydrocarbonaceous feedstock containing straight and slightly branched chain hydrocarbons by contacting the feedstock with a catalyst comprising SAPO-11 and a Group VIII metal.